What's the name of this type of master volume?

It's a reissue, not a real vintage piece. PCB has tiny modern components on it. A real vintage amp would have turrets, or at least much much larger PCB components. Because caps & resistors were much larger back in the day.

Yes Sir, we call them redo loops

I agree

amen ... that can really slow slow slow ....... you down and is frustrating ..... to watch lol

Pre PI Master Volume install is way easier. All you need is single 1MOhm Audio/logarithmic potentiometer and some shielded cable. All you need to do is take the wire from the middle pin of the treble pot and connect it to a slider of the new pot. Then connect that treble pot slider/middle lug to the top lug of the new pot. Last step is ground the lower lug of the new pot. Use shielded wire and ground it only from one side. There you go. There is a lot of discussion on pre and post PI master volume pots. If you are reducing power output stage and want preamp distortion only it will never sound as good as with the distortion from the power stage. Many pro players will tell you that Marshall Plexi tone is that not there when the whole amplifier is creating the distortion and is begging for its life. Non experienced players don’t anticipate that huge Plexi headroom and bone crashing volume that comes with the 100 Watt Plexi and half or God forbid full stack. If you can’t use it get the 50 Watt version and the load box. Or get 18 Watt Bluesbreaker. Some people will advise yellow jackets adapters and EL84 tubes. Some people will note that some amps can work with 2 tubes only. Sure both versions will reduce power but will omit output transformers distortion. Part of the output stage distortion is in the saturation of the output transformer. It adds sort of compression and it shows best on highest volume. Two tubes or 4x EL84 can’t do that. Marshall Plexi Super Lead 100 Watt with the full stack at 10 feet away will scare every player first time you experience it. You will think that you are going to explode. Sadly before that level those amplifiers sound just extremely loud and clean. Even to get them t the edge of the breakup volume levels will be scary high. So choose your amplifier according to your needs and venues where you play. 100 Watt Plexi at home is nice to have but almost useless. You want know what you have until you let it spread its wings.

Nope, Bias remains constant, the signal is just attenuated

They mix each side of the output signal together which cancels it out.

@@d-labelectronics When you turn the master volume towards 0, it lowers the resistance in the bias circuit, which apparently isn't a problem. But it does make me wonder why they had the two 220kΩ resistors in the circuit in the first place.

Yes, That would streamline the operation

Yup I've done it on my own 1987x

Sorry man, never worked on that model

why?

Sorry man, That PCB tech cannot be bypassed with a pot

@@d-labelectronics Thanks Terry, It's the only think I don't like on the amp

eyelet boards are my fav

That's why you use Marshall 🤭

@@Einzatskommando Its true but at least I can derive satisfaction from the fact she's still tight after 50 years.

WHY, they were made to be played LOUD at concerts, they weren't made to be played in bedrooms.

@@jeffreycollins7297 well then you'd better explain that to every other amp in the world that comes with a master volume

@@Wizardofgosz These amps were designed before the concept of a master volume was even invented! 100W Marshalls came about because guitarists of the day were asking Marshall for LOUDER amps....it wasn't until the 70s that master volumes came into being (1976 was when the first Marshall with a master volume came out I think).